Another Tectonic Mechamism Question

[HandwaviumAdAbsurdum]

I've read through your second post now and I have an idea. If you could send post a blank view of this region at 225 Ma, I'll annotate it and give you my thoughts.

[MrBragg]

Sure thing; here are orthographic and equirectangular views of 225 Ma.

225Ma_blank_ortho.png 225Ma_blank_equi.png

[HandwaviumAdAbsurdum]

What immediately caught my attention looking through your post was that your ridges look like they're very close to the Western margin of your Pangaea analogue, and in some cases, have already been subducted. In normal circumstances we would not expect this to be the case, unless your ocean already has an analogue of the Pacific plate at its centre.

The Pacific plate was born at the centre of the Panthalassic triple junction and grew outwards from in at the expense of the Farallon, Izanagi and Kula plates. Your setup looks very similar to the present day situation, with only small vestiges of these ancient plates left as yet unsubducted. If we use this to explain what's going on in your world around 255 million years ago, we can assume that black is on one of these plates.

This setup gives us a big advantage because it means that almost all of the ridges have already been subducted and slab pull effects should begin very soon.

Now, onto the specifica. I don't see why you need a transform fault south of yellow at all. There's no reason it shouldn't be a subduction zone, much like the active margin of the Nazca plate, or indeed the Cocos plate. If anything it's pink's margin that might be a transform fault. As I see it, it has just subducted a ridge (which would have been a northward continuation of the ridge west of black) and now that subduction zone you have there is at a right angle to the remainder of the ridge. What I see coming out of this is very much what we're seeing with the Gulf of California, with a strip of pink's coast being displaced NNW.

That's all my thoughts for now. I'm starting at a temp job today and I won't have much time to post so I should be inactive on the forums for the next few weeks at least. I'll be interested to see this project again when I get back. All the best!


Exterior Ocean.png

[MrBragg]

First off congrats on the job! As far as excuses to not think about fake plate tectonics on the internet go, that's a pretty good one

I spent some time looking at your sketch and, apart from the Baja-like region and the margin with yellow being a subduction zone, I don't think it's so different from the "one plate" sketch I posted a few days ago. This picture makes complete sense at 225 Ma, but it wasn't clear to me how well this would hold up as time goes on. My principal worry was that, after about 150 Ma, yellow and black are moving in near opposite directions, which seemed incompatible with this model as it appeared to require a ridge between those two fragments; it's entirely possible I'm just way out of my depth here and I'm missing something obvious, but I didn't know how to accommodate that in the model. Would that transform region propagate through pink/yellow as they slide along like in the snapshot?

Animation from 225 Ma - 100 Ma
animationEquir_short.gif

Snapshot at 150 Ma
confused.png

In any case, clearly I have a lot to work on here. Thanks for all of the feedback, and hopefully when you get back I'll at least have a little more progress to show!

[MrBragg]

Ok, so I've been hacking away at this for more hours than I should probably admit to in gplates trying to get this inner ocean place to make sense. Despite lots of different contortions, for the life of me I couldn't get the "one plate" model to work given the relative motions of things at the beginning. This is likely a result of my own ignorance, but, basically, I could never reconcile the eventual westerly movement of (the yellow portion of) the northern supercontinent with the east northeasterly movement of the black fragment's plate. This seemed like it had to be a divergent interaction, which I just couldn't come up with a way to accommodate. I could probably modify the starting movements to make things work, but that would basically require starting over from scratch, which I'd love to avoid if possible.

In any case, I did come up with a model that managed to make sense in my head and seems to (hopefully...) work with the continent movements as they are. I used "fake" plates to model the crust being subducted and to make sure the ridges ended up in the right places; an ugly gif of the movements from 225 - 100 Ma is at the end. A few representative snapshots are here, but basically:

- plate A is moving north and being subducted by the northern supercontinent while it subducts plate B to its east
- plate B is moving northwest and is subducted beneath plate A and the northern supercontinent
- plate C is moving northeast and is also being subducted by the northern supercontinent
- plate D and the southern supercontinent are both heading south-ish, though the relative motion between those two isn't something I've worked on yet

225 Ma 225_Ma_plates.png 175 Ma 175_Ma_plates.png

125 Ma 125_Ma_plates.png 100 Ma 100_Ma_plates.png

take_07_animation_slow.gif

How this configuration would come to exist mattered much less than the fact that it made sense to me Until I come to an epiphany and realize something is glaringly wrong--or someone points out glaring wrongness--I think it seems workable up until the 100 Ma collision.

[MrBragg]

I've moved on to starting to think about what's going on in the outer ocean and think I've got a rough outline of justifying how the northern (eastern now) supercontinent fragments. The animation captures the important features, but basically I've set up the outer ocean basin to be something like Tethys, with subduction in the east, a ridge in the ocean (probably also one off to the north, too), and a passive margin at the far western purple continent. At 200 Ma, the ridge jumps within the purple continent and tears a piece off, which subsequently starts a long journey across the ocean toward the subduction zone. While that chunk heads east, a rift forms within the pink eastern continent (~120 Ma) and begins breaking a piece off; depending on the geometry, this could be a back arc basin, or justified by some other yet undetermined mechanism. In any case, that rift eventually goes extinct (~70 Ma) and the rifted fragment hangs out off the coast of pink until it collides with the piece of purple that finally arrives around 30 Ma. This collision reverses the polarity of subduction, putting pink under slab pull and initiating the breakup of the supercontinent. At 16 Ma, pink collides with the offshore fragments and the subduction polarity again reverses, essentially restoring the initial subduction zone with the fragments accreted onto the margin of pink. No longer under slab pull, pink loses most of its westward vector and travels largely south from the northern pieces of the supercontinent.

200 Ma - present
OuterOceanAnim.gif

Eventually I'll need to try and work out the details, especially what's going on off to the north/south, but for the section in the middle I don't think I've violated too many tectonic rules

[MrBragg]

Alright, I took a stab at figuring out the southern ocean and, once I got something that looked reasonable there, I assembled the various regions into a first, rough draft of a planet-wide tectonic model. Voilà!

plates_rough2.png

The arrows indicate rough direction of movement, though the lengths aren't proportional to the speeds. I'm waiting to adjust the coasts / spend time adding in every strike-slip fault / make this actually look half decent until I'm convinced nothing drastic will change, but for a rough tectonic sketch, what do you all think? Anything stand out as grossly--or even modestly--impossible or implausible? Don't hold back, nothing anyone tells me could possibly hurt worse than the pain I've already suffered in gplates!